There are three main technologies that are used for the production of syngas from methane: steam reforming; autothermal reforming and partial oxidation (catalytic and non-catalytic). The most commonly used are autothermal and steam reforming or a combination of the two. Both these technologies require a large proportion of steam to be included with the methane feed to prevent coke formation and reforming catalyst deactivation. In order to achieve high energy efficiency the large amount of sensible and latent heat contained within the steam must be recovered and recycled to the process.
Non-catalytic partial oxidation does not require the high levels of steam but the very high process temperatures (>1200 deg C.) create energy efficiency challenges of their own.
One more recent, non-commercial technology is the catalytic partial oxidation of methane using rhodium catalysts. Rhodium has been found to be highly selective in the oxidation with minimal coke formation allowing the partial oxidation process to be run at much lower temperatures. The process does not require steam to operate, although small quantities (10% vol % of the methane feed) are frequently described as a means of increasing the hydrogen to carbon monoxide ratio in the resultant syngas.
The simplicity of the system, with little or no steam, a lower temperature of operation and a highly active catalyst promises a compact and efficient process that is capable of operating efficiently without extensive steam recycles. However the processes described in the literature prior to U.S. Pat. No. 7,641,888 (Gobina et al.) utilise a pre-mixed feed well within the explosive limits of the gases to produce a selective reaction. U.S. Pat. No. 7,641,888 is incorporated herein by reference. This presents significant safety problems particularly in operation and preheating of the respective feeds. The safety of the reaction relies on the gas velocities being maintained at a sufficiently high speed and that flash back to the inlet point does not occur.
With the invention of a two chamber reactor separated by a porous, catalytic membrane with mixing and reaction taking place simultaneously within the reactor the safety of the system was greatly improved.
However, there is another problem that is found within a fixed bed partial oxidation reactor that is described in the literature but not referred to in the Gobina patent. That is the problem of catalyst overheating. It has since been found that a similar problem can also occur within the two chamber porous membrane reactor described. The steps to overcome this problem within a simple adiabatic reactor are the subject of this patent.
The partial oxidation of methane is a very rapid reaction that takes place at temperatures in excess of 600 deg C. Typically, when performed using a fixed bed of catalyst with a premixed feed comprising methane and oxygen (gas molar ratio of 2:1) the feed or catalyst is preheated to at least 400 deg C. to achieve light off (Journal of catalysis, 249 (2007) pp 380-393 Horn et al.) such that light off of the catalyst is achieved and good selectivity to carbon monoxide is achieved. Once the catalyst is operating at temperature radiation and thermal conduction through the bed, preheating the incoming gas is sufficient to maintain the reaction without preheat. The temperature of the gases passing over the catalyst rapidly rises and under adiabatic conditions (no heat loss) the product gases leaving the reactor can be in excess of 900 deg C. It is also beneficial if the reaction can be performed at elevated pressure since most of the processes that utilise syngas to form another chemical do so at raised pressure and the costs of compressing the component feed streams (comprising methane and oxygen) is less than compressing the resultant syngas. This is principally as a result of the increase in gas volumes that accompany the reaction. The partial oxidation of methane as described in U.S. Pat. No. 7,641,888 (Gobina) is found to have similar characteristics in that it is most beneficially carried out at elevated temperature and pressure.
At temperatures above 600 deg C. the strengths of common materials of construction (e.g. SS 316) for process vessels diminish significantly. In addition material compatibility to avoid corrosion presents problems. Consequently pressure vessels operating at high temperature often require lining with more exotic materials to prevent corrosion and may also require a high strength alloy.
The alternative to construction with an exotic alloy (e.g. 800HT) is to refractory line the inside of the vessel to reduce heat transfer to the pressure containing shelf such that external heat losses results in the shell being maintained at a significantly lower temperature than the gases within the reactor. The demands on the material of construction of the unit are therefore reduced and a cheaper lower specification material can be utilised.
Furthermore, if the reaction can be operated successfully in adiabatic mode then minimal internal pressure containing elements are required within the reactor and use of high alloy materials can be avoided.
In summary the cheapest form of reactor for a high temperature reactor is a refractory lined pressure vessel with no heat transfer to a utility fluid (an adiabatic reactor). This is well known to an engineer who is skilled in the art of reactor design.
There are two main problems that are found in the operation of a fixed bed catalyst with pre-mixed feed for the partial oxidation of methane. The first is the safety issues that are associated with operating in an explosive regime. Some have sought to counteract this by stage wise addition of oxygen to the feed methane requiring a complex series of fixed beds and gas distributors (Conoco U.S. Pat. No. 7,261,751).
The second problem, found with rhodium partial oxidation catalysts in a fixed bed arrangement, is that despite the high selectivity that is achievable with this form of catalyst very high catalyst surface temperatures can form that far exceed the adiabatic reaction temperature. Some have attributed this rise in surface temperature to the super-adiabatic effect that is related to the higher diffusion rates of H2 and H in combustion processes, others have suggested it is a consequence of competing kinetics.
It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems.
It is a further object of at least one aspect of the present invention to provide a process and apparatus for the adiabatic conversion of methane.
It is a further object of at least one aspect of the present invention to provide an apparatus to enhance the recovery of energy produced in the exothermic reaction.
It is a further object of at least one aspect of the present invention to provide an apparatus to enhance the flexibility of handling different pressures and feedstock while keeping high yields.